Here is the text of an article I found after a search a few weeks back - thanks to the bloke who wrote it. Hope it's useful in your case! If you want the pics version I can email it.
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TRANSMISSION SLACK
As a Defender owner with some miles under its belt, I'm sure many of you can relate to the transmission slack/back lash/clunk/clack/...thud......whatever you wish to call it. Like many owners I had adopted a "cautious clutch foot" that enabled slick gear changes with little shunt. However when my wife drove it I'd noticed the shunt getting worse. The prospects of a 3000mile plus drive through France in 5-months time with six people on board jolted me into action to find the cause.
First I approached the obvious. The drive flanges were well worn. At about £10 each I changed to lot. The result was a noticeable improvement.
Then I thought a little more logically....Drive Train .......or ........Suspension??????
Suspension - Springs are 4 year old, all in good condition nothing broken or loose. Shock absorbers a year older so possibly wearing but they work fine and don't bounce. Bushes....hmmmm...look tired in places, look original, some are slightly cracked. A change of bushes is on the cards. More for the vague wallow type effect that the 110 has. Not convinced its a suspension clunk at all.
A Frame ball joint - was advised (LR4x4 forums) to put the transfer box in neutral and apply the handbrake. Get underneath and rock the car (!!!) back and forth to see any wear or movement in the ball joint. I then tried levering it up with a big screwdriver. No movement noted there.
Drive train - This would be more of a problem. The noise seemed to be from the rear, but a bearing type "whirr" was at the front. This is how I tacked them........
• First of all, I took of the front prop, put it in diff lock, and drove up the road...the whirring had gone. Then I replaced the front prop and removed the rear prop...this time the "thud" was much less. This helped pinpoint the problems. Slack at the rear was giving the "Thud" whilst a bearing / UJ at the front was making some noise.
• Rear half shafts - A quick inspection (remove drive flange and pull the shaft out, don't worry. It'll slide back in ok) showed a little wear. But if a little wear is evident on moving the flange back and forth, then I thought that would equal a lot of movement by the time you got the wheel on. They were replaced, and what a difference that made! I couldn't believe how such little wear made such a big "thud". The shaft on the right in the pictures is the new one.
• Prop shaft UJ's - prised with a large lever, All were fine apart from a slightly worn one at the front of the front prop.
• Prop shafts - Hmmm...not easy. Definitely some give on the rear prop. I left it for the time being as there was other matters to consider. The front prop literally fell apart when I removed it. What happens with props is the water gets in the sliding joint. then the splines rust, this lifts the plastic coating that the splines have and it splits resulting in a loose or even seized prop (as informed by commercial prop shaft services who built a new one for me)
• Gear box - my nightmare... proved fine (thank god!) Again with some advice from the chaps on LR4X4 forum I removed the inspection plate at the end of the gear box (where you would fit an overdrive), then removed the 6mm thick bearing retainer plate (lever it of with a driver, careful, it's alloy) then pull out (by hand) the gear and you can see the main shaft. The gear was in mint condition, the bearings were un marked and the shaft was as new!!!! At 165,000 mile this had to be a recon box (yipee!) and the cross drilled gear supported that theory.
• Transfer box - Not as favourable as the gearbox I'm afraid. I already knew I had sideways play on the transmission brake drum (rear output bearing), I also know I have some play on the front output bearing (flange wobbles a good 3mm if you push the front prop). When I had the plate off (above picture) I stuck my hand in the hole and grabbed the transfer box gears, then turned the rear prop back & forth. Lots of movement was obvious, but I don't know if that's normal !!! . Putting 2 and 2 together and getting 5, I'm guessing that the gearbox (well known weak spot LT77) wore out and a recon was fitted, the transfer box was fine at the time though so was just refitted. Hence at 165,000 miles the transfer box gets a little noisy and slack.
• Differentials......hmmmm...don't know what to do with these. A Salisbury on the rear with notable slack, but then its a Salisbury. There apparently notorious for having lots of slack......so I'll leave it and look at the more obvious worn parts first.
Conclusion: Well, as pointed out by many people, the accumulation of a little wear everywhere has the end result of a lot of slack. This is obviously the case, but I would put money on it that the transfer box is guilty of being a little worn shall we say. When the drive flanges were replaced things got better and when the half shafts were replaced things were better still! The plan is to leave the transfer box for the next 4 or five month then re-evaluate it the month before the trip to France. If its got any worse then I'll change it. If its still the same (and I'm skint) I'll just risk it. It just shows it was worth all the investigating, I was about to order a gearbox as I was fully expecting it to be knackered, That's at least £400 I'm not about to spend.
mines been like it for over a year and aint got much worse, i know you can do something to it if aint gone to far? worn splines on out put gear or something? sure some one can clarify that.
